**Low viscosity and high attenuation in MgSiO3 post-perovskite inferred
from atomic-scale calculations**

AM Goryaeva and P Carrez and P Cordier, SCIENTIFIC REPORTS, 6, 34771 (2016).

DOI: 10.1038/srep34771

This work represents a numerical study of the thermal activation for
dislocation glide of the **100** (010) slip system in MgSiO3 post-
perovskite (Mg-ppv) at 120 GPa. We propose an approach based on a one-
dimensional line tension model in conjunction with atomic-scale
calculations. In this model, the key parameters, namely, the line
tension and the Peierls barrier, are obtained from density functional
theory calculations. We find a Peierls stress sigma(p) = 2.1 GPa and a
line tension T = 9.2 eV/angstrom, which lead to a kink-pair enthalpy
(under zero stress) of 2.69 eV. These values confirm that this slip
system bears a very low lattice friction because it vanishes for
temperatures above approximately 500 K under mantle conditions. In the
Earth's mantle, high-pressure Mg-ppv silicate is thus expected to become
as ductile as ferropericlase. These results confirm the hypothesis of a
weak layer in the D '' layer where Mg-ppv is present. Easy glide along
**100**(010) suggests strong preferred orientations with (010) planes
aligned. Highly mobile **100** dislocations are also likely to respond to
stresses related to seismic waves, leading to energy dissipation and
strong attenuation.

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